Most people spend time and money trying to remove moss and vegetation from the roofs of their homes. They do so because they consider it to be dirty and ugly, and because it adds weight to the roof.
But moss has more value in construction than most people realize, particularly when it comes to roofs, and more specifically green roofs. While the idea of 21st century, energy efficient and environmentally friendly green roofs represents a relatively new sustainable technology in the modern Western world, the first green roofs date back to antiquity.
A Quick History of Green Roofs
The first green roofs that we know of were in the Hanging Gardens of Babylon that, of course, is one of the seven wonders of the ancient world. The Vikings also built houses with green roofs – in the form of turf sods or peat. This natural resource continued to be a material that was used for centuries. While the original motivation of those early homebuilders was to use materials that were easily accessible, there’s no doubt that the value of its insulating properties was important too.
We now know that because of “evapotranspiration”, which is the plant’s ability to absorb moisture through its root system and emit moisture via its leaves, the plants (be it moss, grass or anything else) effectively remove heat and provide shade. While conventional roof surfaces can reach a temperature of more than 90 °F or 50 °C (think cat on a hot tin roof), green roofs are cooler than the ambient air.
More recently, Germany became the first new age country to come up with a committed use for green roofs. They have developed modern technology since the 1960s – focusing on public buildings like schools, office blocks, shopping centers and stores, rather than suburban houses.
But funnily enough, the United States’ oldest modern-style green roof is even older than this. Situated on the roof of the Rockefeller Center in New York, this green roof was constructed in 1930. Since then they have become popular in many other US cities, including Portland, Washington and Chicago (which has the greatest number of green roofs in the USA – with more than 600 in 2011). They are also now common in Canada. Canadian, Joy Schmidt, who calls herself the “green roof lady”, has been promoting them for more than 20 years.
The Range of Green Roofs
In its widest sense, a green roof is any roof that incorporates natural plant growth as part of its structure, and includes eco-roofs, garden roofs and any type of “living” roofs. Rooftops that comprise a permanent landscape are also included in the concept.
Extensive Green Roofs
Roofs that copy the original idea of moss or turf surfaces are categorized as being extensive. Moss and some succulents are the most common plant materials used, and considerable research has been carried out to ensure that mosses and succulents planted will adapt to the ecological conditions of the roof site, and grow without care or maintenance.
A German company, Behrens Systemtechnik, began manufacturing moss (specifically bryophytes) roof mats as an alternative to heavier, more expensive slate roofs, in the 1990s. They now have bryophyte plantations where they prepare the moss for green roof construction, and in 2004 began installing their product in Michigan.
Advantages of their product, which may be pitched or laid flat include its ability to:
- Regulate rainwater that falls onto the roof and is then returned to the atmosphere by evapotranspiration.
- Increase biodiversity simply by introducing flora and fauna.
- Minimize the greenhouse effect by balancing temperature – retaining heat in winter and cooling (by evapotranspiration) in summer.
- Improve air quality because the bryophytes use harmful carbon dioxide and produce oxygen. They also trap particles of dust.
- Create a sound barrier for those inside the building.
- Act as a fireproof layer.
Generally these are the advantages of all green roofs, even though construction processes and plants used do vary.
A 2006 study of extensive roofs in London (United Kingdom) by Gary Grant of EcoSchemes Ltd, showed there to be three types:
- Grass roofs dating to the early 1990s
- Mass produced Sedum (a type of succulent) matting for roofs that were popularized in the late 1990s.
- Early 21st century roofs made from brick and crushed concrete that created a habitat where Phoenicurus ochruros (a “rare black redstart” that occurs on derelict sites) would grow and increase biodiversity.
The first grass roofs in London were commissioned as part of a plan to replace man’s footprint with green space that had disappeared as a result of urban development.
Generally, extensive green roofs comprise some sort of structural support, a protective (and waterproof) membrane, and a shallow growing medium that, in the case of bryophytes, is usually laid in much the same way as roll-on lawn.
Where a green roof takes on the form of a roof garden, it is generally described as intensive. Anything that will survive in a rooftop environment can be grown.
Intensive green roofs are only suitable for flat roof surfaces, and the structural support required is far more substantial – due to the heavy weight loads of plants and the fact that a thicker (heavier) growing medium will be needed. Regular fertilizing, watering, pruning and so on is required, just as it would be in any garden. But its benefits are the same (or very similar) to those of extensive green roofs. When constructed on public buildings, they have an added benefit as they may also be used for recreation – a high-rise park or outdoor sitting space.
Semi-Intensive Green Roofs
Joy Schmidt’s Vitaroofs International offers both extensive (with pre-grown vegetation mats) and intensive (with shrubs, grasses and trees for recreational use) roof options throughout Canada. She also offers a compromise semi-intensive system that uses herbaceous plants, indigenous shrubs and coppices, and grasses that need less soil than typical intensive systems, and less maintenance.
Green Roofs Create Healthy Cities
There is no doubt that this is true, and a growing number of cities worldwide are coming to the party. In some parts they are mandatory, including in Toronto, where any industrial, commercial or public building with a roof bigger than 2,000 sq m has to have a green roof. In July 2012 it was reported that there were 135 green roofs in Toronto, and as many under construction.
It makes perfect sense. Quoted in the New York Times in 2005, the executive director of Earth Pledge, Leslie Hoffman, says, “Isolated green roofs are expensive insulation. But when you have a whole community of green roofs, it changes the micro climate of the area and reduces the demand for energy”.
Are Green Roofs a Sustainable Option for Suburban Roofs?
Research shows that all types of green roofs are substantially more expensive than conventional roofs, but because they are so eco-friendly and versatile, the benefits might well outweigh the costs. The question is whether mister average man-in-the-street will be prepared to pay the price.